1. Field of the Invention
The present invention relates to a high frequency line-to-waveguide converter in which a high frequency line, such as a coplanar line or a coplanar line having ground, forming a high frequency circuit and used in a microwave or millimeter wave region is converted into a waveguide, and connection between the high frequency circuit and an antenna or between high frequency circuits is performed through the waveguide, so that mounting of a system can be easily performed.
Besides, the invention relates to a high frequency package for easily connecting a high frequency electronic component used in a microwave or millimeter wave region to a waveguide.
2. Description of the Related Art
In recent years, we enter the advanced information age, and with respect to a high frequency signal used for information transmission, studies have been carried out to utilize frequencies in the range from a microwave of 1 to 30 GHz to a millimeter wave of 30 to 300 GHz, and an application system using a high frequency signal of a millimeter wave, such as an inter-vehicular radar, is also proposed.
In such a high frequency system, there is a problem that since the frequency of a high frequency signal is high, the attenuation of the high frequency signal in a high frequency line constituting a circuit becomes large. For example, in the case where the high frequency line has a microstrip line structure, dielectric loss in a dielectric substrate becomes large in proportion to a frequency (when dielectric loss tangent is independent of a frequency), and conductor loss in the line conductor becomes large in proportion to a square root of the frequency. From this, even when the same microstrip line is used, when the frequency to be used becomes high from 1 GHz to 10 GHz, the dielectric loss becomes 10 times as high, and the conductor loss becomes about 3.2 times as high, and there is a problem that in order to compensate the loss, it becomes necessary to heavily use expensive high frequency components having low noise, high efficiency and high gain, and the system becomes expensive.
It is known that as compared with the high frequency line of the microstrip line structure as stated above, the transmission loss of a high frequency signal in a waveguide is low. For example, the loss of a waveguide WR-28 used in a band of 26 GHz to 40 GHz is about 0.005 dB/cm at 40 GHz, and this is remarkably smaller than the loss of about 1 dB/cm of the microstrip line using an aluminum substrate. This is because as compared with the normal high frequency line (generally designed to have an impedance of 50Ω) by the microstrip line or the like, the impedance of the waveguide is high (although changed according to the frequency, it is designed to be of the order of approximately 500Ω), and in the normal high frequency line, although the contribution of electric field energy transmitted in the dielectric substance is large in relation to the transmitted signal energy, the waveguide has such a structure that air having a dielectric loss tangent of almost 0 is used as the dielectric substance, a current flowing through the wall of the waveguide, which causes relatively low magnetic energy, may be small, and since the current flows through a relatively wide area of the wall of the waveguide, electric resistance becomes small and the conductor loss becomes small.
Besides, waveguides are generally connected to each other by screws. Thus, attachment and detachment can be easily performed. For example, when the waveguide is used for the connection of a high frequency circuit module and an antenna, their respective waveguide ports are used to carry out their respective checks before assembly, and a high frequency front end can be assembled by combining good components with each other, and the manufacture yield can be raised. From these, the front end using the waveguide is conventionally often adopted for transmission between the high frequency circuit module and the antenna, in which a transmission distance often becomes long.
FIG. 14 is a sectional view for explaining a structure of such a high frequency front end. According to FIG. 14, a front end 10 is constructed such that a module 11 and an antenna 12 are connected through a waveguide member 13. The module 11 is mounted on a metal chassis 15 having a waveguide opening 14. Besides, in this front end 10, there is constructed a high frequency line-to-waveguide converter 18 including a microstrip substrate 16 in which a microstrip line as a high frequency line is formed and a waveguide constituted by the waveguide opening 14 and a short circuit termination member 17. A wiring substrate 19 on which a high frequency component is mounted is connected to the microstrip line of the microstrip substrate 16 by wire bonding.
The high frequency line-to-waveguide converter 18 in this front end 10 is of the type in which at a position apart from the short circuit termination surface of the short circuit termination member 17 by a distance of ¼ of a wavelength (guide wavelength), in the waveguide, of an electromagnetic wave excited by a high frequency signal, a probe (a portion where although a line conductor is extended, a ground conductor is not formed) formed on the microstrip substrate 16 is inserted from the side of the waveguide by a length of approximately ¼ of a signal wavelength. This probe functions as an antenna in the waveguide, and radiates a high frequency signal as an electromagnetic wave into the waveguide. The half of the electromagnetic wave radiated into the waveguide is directly transmitted to the lower waveguide member 13, and the remaining half is transmitted toward the upper short circuit termination member 17. The phase of the electromagnetic wave transmitted toward the short circuit termination member 17 is inverted at the short circuit termination surface and is totally reflected. The totally reflected electromagnetic wave is returned to the probe portion, and is combined with the electromagnetic wave directly radiated downward from the probe. At this time, when the distance between the probe and the short circuit termination surface is made ¼ of the guide wavelength, the length of the both way optical path starting from the probe and returning to the probe via the short circuit termination surface becomes the ½ wavelength, and the phase of the electromagnetic wave reflected at the short circuit termination surface becomes opposite to that of the electromagnetic wave directly radiated from the probe by the optical path difference. Eventually, the phase of the electromagnetic wave reflected at the short circuit termination surface is inverted when it is reflected at the short circuit termination surface, and further, the phase is reversed by the optical path difference, and becomes the same as the phase of the electromagnetic wave directly radiated downward from the probe, and the electromagnetic wave is transmitted to the lower waveguide member 13.
At this time, in order to cause the probe to function as the antenna, the length of the probe inserted into the waveguide is required to be made exactly ¼ of the wavelength of the transmission line. Besides, in order to cause the phase of the electromagnetic wave radiated from the probe upward and reflected at the short circuit termination surface to become the same phase as the phase of the electromagnetic wave radiated downward from the probe, the distance between the probe and the short circuit termination surface is required to be made exactly ¼ of the guide wavelength. Accordingly, the characteristic is greatly changed by the insertion position of the microstrip substrate 16, which functions as the antenna, into the waveguide, and the relation between the position of the microstrip substrate 16 and the position of the short circuit termination surface of the short circuit termination member 17.
Since the high frequency line-to-waveguide converter 18, together with the wiring substrate 19, is constructed on the metal chassis 15 by assembly, there is a problem that in the case where conversion loss of the high frequency line-to-waveguide converter becomes large by position shift of the respective members, the assembly becomes poor, and all of the used members become wasteful. Besides, the related art is disclosed in WO96/27913 and Japanese Unexamined Patent Publication JP-A 2001-177312 (2001).
FIG. 15 is a sectional view for explaining a structure of a high frequency line-to-waveguide converter. According to FIG. 15, a front end 20 is constructed such that a high frequency package 21 is connected to an antenna 22 through a waveguide 23. The high frequency package 21 is constructed such that a conversion substrate 26 having a built-in waveguide converter 25 is joined to a metal base 24. The waveguide converter 25 converts a plane circuit 28 for transmitting a high frequency signal processed by a high frequency electronic component 27 mounted on the high frequency package 21 into a waveguide mode 31 through a slot 30 formed in a ground layer 29 in the inside of the conversion substrate 26.
In this high frequency package 21, it is necessary to provide the area for mounting of the high frequency electronic component 27, together with the waveguide converter 25, in the conversion substrate 26, and there is a problem that in the case where the number of parts of the high frequency electronic component 27 is increased, the size becomes large, and warp or fracture can occur due to the mismatch in thermal expansion between the conversion substrate 26 and the metal base 24 at the time of assembly of the package. Besides, the related art is disclosed in U.S. Pat. No. 6,239,669.
In order to solve the problem as stated above, for example, WO96/27913 proposes a microstrip-waveguide transition including a microstrip line formed on an upper surface of a dielectric substrate and a slot formed in a lower ground conductor layer and functioning as an antenna. In the microstrip-waveguide transition proposed by WO96/27913, the thickness of the dielectric substance from the slot to a waveguide is made ¼ of a signal wavelength of a high frequency signal. This is such that a difference in impedance between the slot and the waveguide is adjusted by a ¼ wavelength matching device of the dielectric substance.
According to this structure, an electromagnetic wave radiated from the slot and reflected at a boundary between the matching device of the dielectric substance and the waveguide is reflected at the ground conductor layer in which the slot is formed, and is again returned to the boundary between the matching device and the waveguide. At this time, when the thickness of the matching device is made ¼ of the signal wavelength, an optical path difference between the electromagnetic wave (reflected wave), which is reflected at the boundary and is again returned, and the electromagnetic wave (direct wave) directly transmitted from the slot to the boundary becomes ½ of the signal wavelength, and the phase is inverted when the reflected wave is reflected at the ground conductor layer, and accordingly, the direct wave and the reflected wave have the same phase at the boundary to intensify each other, and are transmitted to the waveguide.
According to this conversion structure, although the conversion characteristic is greatly changed by the thickness of the matching device, in this case, since the matching device is integrally constructed in the dielectric substrate, it becomes possible to lessen variation in the thickness of the dielectric substance, and variation in the conversion characteristic can be made small. Besides, when the dielectric substrate at the microstrip side is covered with a cap, it also becomes possible to airtightly seal the microstrip side at the same time as the conversion into the waveguide.
In this structure, electromagnetic coupling between different layers is used for coupling of the high frequency line and the slot. This electromagnetic coupling, together with the foregoing matching device, plays a main role in the conversion operation. However, the characteristic of the electromagnetic coupling is changed by the size of the slot and the length of a stub (a portion where the high frequency line protrudes from the slot), that is, the relative positional relation between the high frequency line and the slot. Accordingly, in this structure, the conversion characteristic is greatly changed by the size of the slot and the length of the stub, and since the high frequency line and the slot are disposed in the different layers, there is a problem that the length of the stub determined from the relative positional relation between both is apt to vary, and the conversion characteristic is apt to change.
Besides, in this structure, since the slot is placed in the inside of the dielectric substrate, there is a problem that it is difficult to check the length of the slot, the width of the slot, and the length of the stub from the outside, and it is also difficult to stabilize the characteristic by making a check.
In order to solve the problem as stated above, for example, a high frequency line-to-waveguide converter is conceivable in which a slot functioning as an antenna is formed at a tip of a coplanar line on a surface of a dielectric substrate, a waveguide is connected to a rear surface of the dielectric substrate at a position opposite to the slot, and a shield conductor part for connecting the waveguide and a ground conductor layer of the coplanar line is provided along an opening of the waveguide. The coplanar line is constituted by a line conductor and ground conductor layers disposed at both sides thereof, and the ground conductor layers in this case function as the ground of the coplanar line, and further function also as reflecting plates for again reflecting an electromagnetic wave (reflected wave) radiated from the slot, reflected at the boundary between the dielectric substrate and the waveguide and returned to the slot side. According to this converter, when the distance from the slot to the boundary between the dielectric substrate and the waveguide is set to ¼ of the wavelength of the electromagnetic wave transmitted through the dielectric layer, an optical path difference between the reflected wave, which is radiated from the slot, is reflected at the boundary between the dielectric substrate and the waveguide, is again reflected at the ground conductor layer and reaches the boundary, and the electromagnetic wave (direct wave) directly transmitted to the boundary from the slot becomes equal to ½ of the wavelength of the electromagnetic wave, and the phase of the magnetic field of the reflected wave is inverted when it is reflected at the boundary between the dielectric substrate and the waveguide, and accordingly, the direct wave and the reflected wave have the same phase at the boundary to intensify each other, and are transmitted to the waveguide. That is, the dielectric substrate intervening between the slot and the waveguide and having the thickness set to ¼ of the wavelength of the electromagnetic wave functions as a matching device of the slot and the waveguide whose impedances are different from each other.
However, in this structure, since the coplanar line is in contact with the matching device of the dielectric substrate, part of the electromagnetic wave of the signal transmitted through the coplanar line is distributed in the matching device, and this generates an unnecessary electromagnetic wave distribution (here, called a mode) in the matching device, and there is a fear that the transmission of the high frequency signal to the waveguide is impeded. For example, immediately under the line conductor of the coplanar line, the magnetic field by the signal becomes parallel to the surface of the dielectric substrate. This magnetic field excites a TM mode as a resonant mode at the time when the matching device is made the dielectric waveguide, and the signal energy of a TE mode as a transmission mode shifts to the TM mode and resonates, and the signal is reflected, and accordingly, there is a case where the conversion into the waveguide can not be excellently performed.
In order to solve the problem as stated above, it is conceivable that for example, the conversion substrate 26 including only the waveguide converter 25 is fabricated, and is connected to the metal base 24. By doing so, it becomes possible to lessen the conversion substrate 26, the residual stress after the assembly due to the mismatch in thermal expansion between the conversion substrate 26 and the metal base 24 becomes low, and it is possible to prevent the warp or fracture of the high frequency package 21.
However, in this structure, when the upper surface of the conversion board 26 and the upper surface of the high frequency electronic component 27 are made the same surface, although the respective signal lines can be connected by wire bonding or ribbon bonding at a relatively short distance, since the thickness of the conversion substrate 26 including the waveguide converter 25 is generally overwhelmingly thicker than the thickness of the high frequency electronic component 27 used in the microwave or millimeter wave range, a connection distance between grounds at the respective lower surfaces becomes longer than a connection between signal conductors, and there is a case where the phase of the electric potential of the signal conductor deviates from the phase of the electric potential of the ground conductor at the connection part, and the high frequency signal can not be excellently transmitted.